(1) Field of the Invention
This invention relates to an alkaline storage cell comprising a positive electrode of nickel, a negative electrode and an electrolyte, especially to an improvement of the positive electrode and the electrolyte.
(2) Description of the Prior Art
Recently, along with multi-functionization of equipment using a storage cell, a storage cell having larger capacity has been demanded. In response to this demand, the followings have been proposed.
(a) As disclosed in Japanese Patent Publication Kokai No. 62-37874, the utilization factor of an active material of a positive electrode is improved by forming a layer of conductive cobalt oxide between a surface of a plaque and the active material. In this construction, the utilization factor of nickel hydroxide as the active material, exceeds 90% because the discharge characteristic is improved mainly at the final stage of a discharge cycle.
(b) As disclosed in Japanese Patent Publication Kokai No. 62-71168, a porous metal plaque filled with an active material is immersed in a solution of acid cobalt salt and alkali-treated, whereby to form cobalt hydroxide on a surface of the active material.
(c) As disclosed in Japanese Patent Publication Kokai Nos. 63-211563 and 63-224159 and Japanese Patent Publication No. 59-10538, the characteristic of a nickel positive electrode is improved by adding lithium hydroxide to an electrolyte so that lithium hydroxide can act on the positive electrode. In the above nickel positive electrode, the cobalt compound does not independently constitute a layer but forms a solid solution or a mixed crystal together with nickel. In this construction, the utilization factor of an active material at the first several cycles is improved. Especially, if the electrolyte is produced by adding lithium hydroxide to a solution mainly comprising potassium hydroxide, the utilization factor of the active material at the first several cycles exceeds 90%.
However, (a) and (b) can further be improved. Since a commonly-used nickel sintered positive electrode already has high conductivity in its plaque, its discharge capacity is inevitably high. Therefore, remarkable effects cannot be obtained by improving the utilization factor of the active material.
(c) has the following problem. Lithium hydroxide promotes charging, whereby generating higher-order nickel oxyhydroxide (.gamma.-NiOOH) than tervalent nickel oxyhydroxide (.gamma.-NiOOH). .gamma.-NiOOH temporarily shows high capacity because it is high in order, but is hard to discharge because of its comparative inactiveness. Therefore, .gamma.-NiOOH is accumulated as a non-discharged active material while charge/discharge is repeated, which decreases the cell capacity. Further, since the accumulated non-discharged active material is low in density, the electrode plate is weakened. As a result, the active material comes off from the electrode plate, further decreasing the cell capacity.